This invention relates generally to image projection systems, and is concerned more particularly with systems that include a spatial light modulator (SLM) for imparting image information to a projected light beam. Systems of this type typically are used for large screen televisions, which are often referred to as xe2x80x9celectronic projectorsxe2x80x9d.
In a typical electronic projector, the SLM may be a liquid crystal device (LCD) comprising a matrix of individually addressable liquid crystal pixels. Each pixel can be switched between a transmissive mode in which incident light from the light source passes through the pixel and is projected, and a non-transmissive mode. In the non-transmissive mode, the light may be absorbed or reflected away from the projection lens. In any event, each pixel has an xe2x80x9conxe2x80x9d state and an xe2x80x9coffxe2x80x9d state. By appropriately controlling the pixels in accordance with stored data, image information is imparted to the projected light beam.
U.S. Pat. No. 5,584,991 (Levis et al.) discloses an example of a LCD projection system.
Another example of an SLM that includes an active matrix of pixels is known as a deformable (or digital) mirror device (DMD). In this case, the matrix comprises an array of tiltable mirrors, each of which positioned on a hinge element above electrodes that allow the mirror to be electrostatically deflected between two positions. The device is operated in a binary manner so that each mirror switches between an xe2x80x9conxe2x80x9d state and a xe2x80x9coffxe2x80x9d state. The mirror angularly deflects the incident light beam so that the beam is either reflected through the projector optics, or not.
U.S. Pat. No. 5,061,049 (Hornbeck) discloses an example of a DMD device, which is hereby incorporated in its entirety by reference. U.S. Pat. No. 5,535,047 (Hornbeck) discloses further improvements to the DMD device of U.S. Pat. No. 5,061,049 and is hereby incorporated in its entirety by reference.
Known projection systems for producing 3D images in which light from a light source is modulated by an SLM and then polarized suffer the disadvantage that there is often a limit on the amount of light flux that can be directed onto the SLM. This limit is caused by, for example, limitations associated with the heating effect of the radiant flux, or saturation due to high luminous flux. This limit prevents increasing the light flux directed onto the SLM to overcome the losses introduced by the polarization of the light leaving the SLM.
Another problem with SLMs is that there is a tendency for some of the incident light to be scattered or reflected, which reduces the overall contrast of images projected onto the screen.
An object of the present invention is to address these disadvantages with the aim of improving the contrast of the projected images.
The present invention addresses the improvement of contrast in electronic projectors utilizing DMDs by reducing the amount of scattered light that reaches the projection screen. The contrast improvement results from polarizing the input light to the DMD and selecting appropriate materials and surface treatments for the projector""s components so that the unwanted reflection and diffraction effects that produce scattered light also depolarize or change the orientation of the polarization of the light. This allows a second polarizer at the output of the projection optics to discriminate between the desired light and the unwanted scattered light. For 3D applications, polarizing the light before the DMD reduces the heat load on the DMD, this allows higher illumination levels to be used to compensate for the loss of brightness due to polarization. The DMD can be operated within its stress ratings and illuminated to the maximum level by polarized light.
Generally speaking, the first polarizer means pre-polarizes or xe2x80x9ccharacterizesxe2x80x9d the light. Light that is subsequently scattered within the projector and depolarized will be partially blocked (up to a maximum of 50%) by the second polarizer means. Accordingly, the contrast ratio of the projected image will be increased by a factor of up to 2.
This is distinct from systems such as LCD projectors where the use of polarized light is essential in order to obtain pixel intensity control from the electrically alterable polarization property of the liquid crystal medium. In LCD projectors, the input light to the LCD is polarized, either prior to the LCD or by a polarizer that is integral to the LCD assembly. A second polarizer then analyzes the output light of the LCD according to the amount of alteration performed by the LCD on the input polarization.
Projectors based on DMD devices do not require polarized light. The use of polarization with DMD devices has been thought of as undesirable as it reduces by half the amount of light that the projection system can deliver to the screen. However, in a system for projection of 3D images, two sets of images are produced, one for each eye, and are characterized or coded by orthogonally polarized light. In a traditional system, the light is usually polarized after the projector lens, resulting in an efficiency loss of roughly 50%. This loss of efficiency requires high input light levels to be used, which can lead to excessive heating of the DMDs. This invention avoids this excessive heating by polarizing the light before the DMDs in the projector, therefore reducing the radiant flux and associated heating of the DMD.
An advantage of the invention is that it is somewhat easier to characterize the unwanted xe2x80x9cnoisexe2x80x9d (scattered light) by polarization than by trying to characterize the signal in some other way. Inefficiencies in the polarizing material are below significance since the amount of noise is relatively small compared to the signal. Inefficiencies such as inequities in performance depending on wavelength or angle of incidence can be tolerated much more readily when applied to the noise component of the overall signal.
Additional significant improvements in the contrast ratio of the projected image are obtained by controlling the surface properties of materials used within the projector where light may be scattered so that those surface properties will further rotate the polarization or depolarize the stray or unwanted light when it is reflected from those surfaces.